1. Field of the Invention
The present invention relates to an improvement on a method of manufacturing wire for use in a wire saw used for slicing, for example, a semiconductor ingot and to an improvement on wire for use in a wire saw.
2. Description of the Related Art
Conventionally, there has been known a wire saw for slicing many wafers from a semiconductor ingot simultaneously. In the wire saw, a semiconductor ingot is pressed against a fine wire such as a hard steel wire or a piano wire which is being fed reciprocatively or unidirectionally while abrasive grain slurry is fed to a contact portion between the wire and the ingot.
A wire made of an iron or iron alloy wire described in, for example, JIS G 3506 and JIS G 3502 is used as a starting wire which is formed into a wire for use in a wire saw. Such a starting wire is generally plated with copper or copper alloy.
The reason for plating a starting wire with copper or copper alloy is to impart a corrosion preventive effect and to impart a lubricating effect during a drawing process in which the starting wire is passed through several dies having different sizes of holes so as to be drawn in several steps. When a starting wire not plated with copper or copper alloy is drawn through a die hole to be drawn, not only does a drawing rate significantly decrease due to poor lubrication between the starting wire and the die, but also there are formed on the wire surface many scratches, which impair quality characteristics that a wire saw wire is required to have, such as tensile strength. Thus, this lubricating effect is particularly important.
As shown in FIG. 7, a starting wire plated with copper or copper alloy is fed from a starting wire reel 51 and drawn into a wire having a predetermined diameter in a finish drawing step. The thus-drawn wire is taken up onto a product take-up reel bobbin 52 for later use in a wire saw.
However, when such a wire plated with copper or copper alloy is used to slice a semiconductor ingot, not only does copper or copper alloy plating exfoliate from the wire surface during slicing and accumulate in collected abrasive grain slurry, but also copper or other metallic impurities contained in copper or copper alloy plating directly contact the sliced surface of the semiconductor ingot, resulting in contaminated product wafers. Copper has a greater coefficient of diffusion within silicon than does iron, which is the material used for the wire. Therefore, when semiconductor silicon is contaminated with copper, copper is highly likely to diffuse from the sliced surface into the bulk of a silicon wafer due to heat generated by slicing friction between a wire saw and a silicon ingot. Thus, product wafers are difficult to be decontaminated by merely cleaning the surface thereof.
There is known a method of drawing wire not plated with copper or copper alloy as disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 5-16066, wherein a starting wire of high-carbon steel is decarburized before drawing so as to convert the region ranging from the surface to a depth of tens of micrometers into soft ferritic structure. This method improves drawing workability, but has a drawback of yielding instable mechanical characteristics due to decarburization. Accordingly, this method is not applicable to manufacture of a wire saw wire which is required to have a certain high tensile strength.
Thus, there has been eager demand for a method of manufacturing at low cost wire for use in a wire saw without adversely effecting quality characteristics which the wire saw wire is required to have and without raising a problem such as metal contamination during a slicing step.